This invention relates to a method and apparatus for making moisture-absorbent substrates, and more particularly, to a method and apparatus for interspersing highly moisture-absorbent particles throughout a predetermined portion of a fibrous material pad in a non-woven product.
Hygenic articles such as disposable diapers, sanitary napkins, incontinence pads and sick bed sheets must have a high absorption capacity to effectively retain eliminated body fluids for acceptable periods of time. Early hygenic articles of this type employed cellulose wadding, fluff cellulose or absorbent cotton as absorbent materials. The problem with these materials is that their moisture-retaining capacity is relatively small compared to their volume. In order to improve the moisture-retaining capacity of hygenic articles made from these materials, the volume of such absorbent materials in the hygenic article must be increased. This produces a bulky product which is unacceptable in many hygenic articles, particularly sanitary napkins.
In an effort to reduce the volume and size of hygenic articles, and increase their absorbent capacity, fluid-absorbent substrates have been developed in which highly absorbent materials are combined within the fiber structure of cellulose fluff, wood pulp, textile fibers or other non-woven, fibrous materials. Many substantially water-insoluble absorbent polymers having a high capacity for absorbing water and body fluids have been developed in recent years for enhancing the moisture-absorbent capability of hygenic articles. These polymers are partially or wholly synthetic and are commercially available in fine grain, particulate form. See, for example, U.S. Pat. Nos. 3,997,484; 3,661,815; 4,117,222; and 3,936,441.
One system for incorporating a moisture-absorbent core or laminate in a diaper is disclosed, for example, in U.S. Pat. No. 3,984,272. The system of this patent includes a forming chamber having an inlet and outlet which is connected by a feed conduit to a source of fibrous material such as finely ground wood pulp. A perforated conveyor is movable through the forming chamber between its inlet and outlet above a duct located at the base of the forming chamber. The duct is connected to a source of vacuum which is operable to create a negative pressure within the forming chamber.
The fibrous material or fibers are injected into the forming chamber through the feed conduit and drawn onto the perforated conveyor by operation of the vacuum source. The fibers form a non-woven pad atop the conveyor whose density is controlled by the vacuum pressure and feed rate of the conveyor. The pad is then transmitted to a leveling or scarfing roller near the outlet of the forming chamber which is operable to remove at least a portion of the fibrous material at the top of the non-woven pad to produce a non-woven pad of uniform thickness. The non-woven pad is then transmitted by the conveyor through the outlet of the forming chamber for subsequent operations to form the completed hygenic article.
In addition to incorporating a moisture-absorbing pad or laminate in a diaper to produce a diaper having improved moisture-returning capabilities, there are also prior art methods for combining highly moisture-absorbent material with a non-woven pad. In one method, the moisture-absorbent material is injected into a feed conduit which directs the fibrous material into a forming chamber in an apparatus of the type disclosed in U.S. Pat. No. 3,984,272, as discussed above. The moisture-absorbent material and fibrous material are intermixed within the feed conduit to completely intersperse the moisture-absorbent material throughout the fibers prior to introduction into the forming chamber. This produces a non-woven pad atop the conveyor within the chamber in which the moisture-absorbent material is present throughout the entire thickness, width and length of the non-woven pad.
One problem with the above-described method is the loss of moisture-absorbent material through the perforated conveyor in the forming chamber. As the fibers and moisture-absorbent material mixture is drawn onto the perforated conveyor to form the non-woven pad, moisture-absorbent material at the lower portion of the non-woven pad is drawn through the conveyor into a filtering-reclamation system. A loss of about 20% of the moisture-absorbent material is not uncommon. Additionally, the moisture-absorbent material is difficult to contain, even within the filtering system, and environmental contamination can result.
A second problem with this method involves damage to the apparatus used in subsequent operations to form the finished hygenic article, particularly cutting devices. For example, in manufacturing disposable diapers, the non-woven pad must be cut to length and formed with leg holes by the operation of die cutters or other cutting devices. It has been found that the presence of moisture-absorbent material throughout the entire pad structure rapidly dulls die cutters which reduces their effective life substantially.
A third problem with this method is that the moisture-absorbent material is distributed throughout the non-woven pad across its entire length and width. This produces substantial waste because in subsequent forming operations the non-woven pad is cut to the desired length of the hygenic article. In addition, the application of moisture-absorbent material across the entire width of the non-woven pad may be unnecessary for some types of hygenic articles, particularly disposable diapers where the leg holes are cut at the edges of the layer.
Another prior art method of combining moisture-absorbent material with the non-woven pad described above comprises applying moisture-absorbent material to the top surface of the non-woven pad downstream from the leveling or scarfing roller and outside of the forming chamber. This has the advantage of eliminating waste of the moisture-absorbent material since there is no loss through the perforated conveyor. Wear on die cutters is still a problem, but not as serious a problem as the other method described above since only the top surface of the product contains the moisture-absorbent material.
One disadvantage of this method is that the moisture-absorbent capacity of the non-woven pad is substantially limited because the moisture-absorbent material is concentrated on the top of the pad. This causes so-called "gel blockage" wherein the moisture-absorbent material at the top of the non-woven pad becomes saturated with fluid and prevents the wicking or transfer of moisture to the remaining portion of the pad. As a result, the fluid is retained at the surface of the pad in contact with the wearer of the hygenic article causing discomfort. Hygenic articles made in accordance with the first method described above also exhibit this problem, to a lesser extent, because at least some of the moisture-absorbent material is located at the top of the non-woven pad.
A second disadvantage of this second method, and for that matter the first method described above, is migration of the moisture-absorbent material, particularly if it is combined with the non-woven pad in particulate form. The moisture-absorbent material in both methods of application is located, at least to some extent, near or at the top of the non-woven pad. In particulate or granular form, the moisture-absorbent material can be dislodged from the type of pads which are not sealed at the ends.
A third disadvantage of this second method is that application of the moisture-absorbent material atop the non-woven pad is performed outside of the forming chamber. This requires some type of collection system to capture the oversprayed material and prevent it from escaping to the environment. This adds expense to the system, and, if not properly designed, can lead to environmental contamination from the uncollected moisture-absorbent material.